Technical Field
The present invention relates to fiber optic sensor technologies that utilize dependency of the frequency shift in stimulated and spontaneous Brillouin scattering on strain and/or temperature, and particularly to a Brillouin scattering measurement method and a Brillouin scattering measurement system that utilize Brillouin backscattered light obtained by launching an optical pulse into one end of an optical fiber.
Description of the Related Art
Conventionally, two main kinds of measurement techniques have been proposed and/or demonstrated for improvement in accuracy, particularly, in spatial resolution in distributed strain measurement and/or distributed temperature measurement with an optical fiber. One technique performs the measurement using both end of an optical fiber and is referred to as “Brillouin optical time-domain analysis” (BOTDA) that performs time-domain measurement or as “Brillouin optical correlation-domain analysis” (BOCDA) that performs correlation domain measurement. The former includes a measurement technique referred to as “phase shift pulse Brillouin optical time-domain analysis” (PSP-BOTDA) that obtains a high gain but reaches only sub-meter spatial resolution. The latter includes a measurement technique that is superior to the former because of a millimeter-order spatial resolution; however, the technique is typically applied only to a limited range measurement and needs a delay line fiber of more than twice as long as the measuring optical fiber.
In contrast to the above, there has been another measurement technique referred to as “Brillouin optical time-domain reflectometry” (BODTR), which uses the Brillouin backscattered light extracted by launching an optical pulse into only one end of an optical fiber. This technique, in principle, detects physical quantity such as strain from change in the frequency shift of a Brillouin scattered light and a position from the light round-trip time between the launch point and a scattering point. However, there have been few conventional reports on BOTDR that demonstrate high spatial resolution, in particular, for a case of long measurement range. Furthermore, in BOTDR, due to the uncertainty relation, it is indicated that there is a limitation on simultaneous enhancement of the spatial resolution and the frequency resolution in a single measurement; hence it is said that combination of a plurality of measurements is necessary (see K. Nishiguchi et al., “Synthetic approach for Brillouin optical time-domain reflectometry”, 42nd ISCIE International Symposium on Stochastic Systems Theory and Its Applications (SSS'10), 2010, pp. 81-88).
Representative measurements that achieved enhancement of spatial resolution by BOTDR include the following two examples. A first example is referred to as “double-pulse Brillouin optical time-domain reflectometry” (DP-BOTDR) (see Y. Koyamada, et al., “Novel Technique to Improve Spatial Resolution in Brillouin Optical Time-Domain Reflectometry”, IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 19, NO. 23, Dec. 1, 2007). A second example is referred to as “synthetic Brillouin optical time-domain reflectometry” (S-BOTDR) (see JP5,493,089 B2), which is a technique that combines a plurality of measurements and synthesizes the spectra obtained by the measurements in order to overcome the limitation due to the uncertainty relation.
The first example demonstrated a high spatial resolution for the first time. The example used two short optical pulses as a probe. Specifically, two short pulses having comparable durations were launched into an optical fiber with an interval of approximately 10 ns, and produced Brillouin backscattered signals were passed through a filter matched to an envelope of the two pulses, whereby a spatial resolution of 20 cm was demonstrated.
The second example used as an optical pulse probe a combination of long duration and small amplitude pulses and a short duration and large amplitude pulse. Specifically, by generating four types of probes combined with the large amplitude and short duration pulse and the small amplitude and long duration pulses modulated in a quadri-phase shift keying, an excellent spatial resolution of 10 cm was demonstrated in a measurement range over several tens of kilometers.
However, in the DP-BOTDR measurement disclosed by Y. Koyamada, et al., since the Brillouin frequency shift was evaluated on the basis of an envelope of the two pulses having a single phase and comparable durations, the evaluated frequency shift contained a large error; hence it is difficult to determine the true frequency shift. In contrast, in the S-BOTDR measurement, although a Brillouin frequency shift can be evaluated without error since the true value of the frequency shift is theoretically calculated, the Brillouin frequency shift needs to be evaluated by synthesizing the detected signals using four types of phase modulations; hence the S-BOTDR measurement system is complicated and takes time in measuring the true value.
The present invention is made in light of the above described problems and aimed at providing a Brillouin scattering measurement method and a Brillouin scattering measurement system that are capable of achieving, more conveniently than using a S-BOTDR, an excellent spatial resolution in a long range measurement, by utilizing a BOTDR that uses two types of optical probes respectively composed of short pulses and adjacent long pulses with bi-phase and zero-phase modulations and uses cross-correlations between signals sampled with window functions of narrow and wide widths from signals detected from Brillouin backscattered lights produced by the probes.